Bacterial panicle blight (BPB) is a weather-associated disease. Environmental conditions in 2010 and 2011 were favorable for the disease to be severe enough to cause devastatingyield loss in susceptible varieties. Panicle symptoms (Photo 1) typically develop late in the season which makes predicting disease outbreaks difficult. Heavily-infected panicles remain upright due to lack of grain fill and consequently both grain yield and milling quality are decreased. Presently there are no chemical options registered in the U.S. to protect or salvage the crop from the disease.Although Jupiter and hybrids are moderately resistant to BPB, most of our non-hybrid commercial varieties are susceptible. Therefore, field studies were conducted in 2012 to examine whether a range of planting dates could be used to manage this disease.
Three planting dates were picked from the third week of March to May (March 20 as early, April 24 as average, and May 24 as late). The bacteria causing panicle blight disease not only are seed-borne, but they can also live in soil or crop residue. In our experiment, an artificial inoculation method was developed in the laboratory to inoculate seeds with a bacterial suspension in order to obtain uniformly infected seeds. Two rice varieties were used: Bengal (susceptible variety) and Jupiter (moderately resistant variety). Treatments were randomized and the experiment replicated four times. Plots across all three planting dates were managed similarly with regards to fertility, water, seeding rate, and herbicide application. To obtain measurable disease data, upright panicles with typical disease symptoms were counted as 100 percent infected but those with symptoms on the lower half the panicle as 50 percent infected. Moreover, yield and milling quality data were collected for all three planting dates.
For the March planting, plots averaged 0.37%infection for Jupiter and 0.44% for Bengal (very low levels of disease for both varieties planted early). For the April planting, infection was still low, averaging 0.43 %for Jupiter and 0.69 %for Bengal.However, for the May planting date bacterial panicle blight exploded in the plots, averaging 49.4 %of panicles infected for Jupiter and 99 percent for Bengal(Fig. 1). Likewise, yield and milling quality were highly affected in May planted plots. A 41%yield loss was calculated for May-planted Bengal and 33 %yield loss for Jupiter when compared to the April planting. Yield comparisons were not made with the March-planted plots because the plots planted in March were damaged by bird feeding both before emergence and after heading (Fig. 2). The extent of the bird damage was not measured. Head rice yield were similarly reduced for May-planted plots compared to April-planted plots with 25 percent and 22 percent reductions for Bengal and Jupiter, respectively.
In 2012 conventional (i.e. non-hybrid) cultivars known to be susceptible to BPB comprised about 65 % of the rice planted in Arkansas. Jupiter, a moderately resistant variety, was planted on 10 percent of the acreage. The remaining acreage was cultivated with hybrids. Although the rice season was hot and dry, there were very low levels of BPB incidence in production fields. The low disease situation can possibly be attributed to early planting which agrees with our research study from last year. However, these data are from a one season of testing and we also need to remember that BPB is a weather associated disease. In other words, if the weather conditions are favorable, disease severity may be high even if the rice is planted early. We remain uncertain about weather factors and the exact developmental stage of the crop needed for a high disease situation.
- Regardless, of the unknown factors, we believe “early planting” would help rice “escape” the severe heat of July and early August. This is predicted from our observation in the previous years and from our research data last year.
- In addition, to early planting, we encourage rice producers to manage their fields with adequate water and potassium fertilizer. Excessive nitrogen and seeding rates may aggravate the disease situation.
- Rice after rice cultivation can build up the bacterial population in soil and residue and may serve as source of inoculum even if the seed sources are clean. So, crop rotation can be important to break the disease cycle.
- Remember, there are no chemical options to control BPB of rice!